This is due the fact that DVD discs have much, much bigger dynamic scale than for example audio CDs -- therefor the audio seems to be much lower when you watch VCD as your DVD player doesn't necessarily adjust the dynamic range correctly for those discs and therefor some people prefer to "normalize" the audio track -- i.e. to increase the volume of the audio track. The negative aspect in this practice is that by doing this, you reduce the dynamic scale of the audio and the huge volume level difference in for example speech and car blasts isn't that huge as it was originally intended to be. You also lose some quality of the higher volume scenes by doing the normalizing.